Recently, optical Differential Phase Shift Keying (DPSK) modulation has received considerable attention by the telecommunications industry primarily due to its increased sensitivity over commonly used On-Off-Keying (OOK) and its reduced peak power, which mitigates nonlinear effects in fiber-optic applications. This has led to demonstrated utility in long haul applications, with experiments confirming more than 3 Tb/s capacity using 80 Wavelength Division Multiplexed (WDM) DPSK channels. It is expected that the first wideband telecommunications fiber optic links using WDM DPSK will be deployed by 2006. DPSK is also an attractive modulation format for high-rate spectrally efficient Free Space Optical (FSO) links because the increase in sensitivity over OOK allows for a corresponding reduction in costly transmitter power.
The benefits of optical DPSK come at the cost of increased complexity, requiring a phase modulator in the transmitter and an optical interferometer and balanced detection in the receiver in order to derive maximum benefit. Of these elements, the interferometer is the most technically challenging and the least mature. Control hardware is necessary to ensure stable operation, which requires that the arms of the interferometer be stable to small fractions of a wavelength. As a result, carefully designed thermo-mechanical packaging is necessary in addition to stabilization electronics, adding to size, weight, power, and cost.